mtree: Remove stray atm directory

ATM support for netgraph was removed in af0cc0b22362 ("NgATM: Remove
netgraph ATM support")

Remove the directory from the mtree specification.

Reviewed by: emaste
Fixes: 21735dfaebdf ("include: Remove no longer existing netgraph/atm")
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D53260